In Riemannian geometry, the "lowering indices" operator is denoted by $\flat:TM \to T^*M$ and the "raising indices" operator by $\sharp:T^*M \to TM$. These isomorphisms are sometimes referred to as musical isomorphisms, as stated on Wikipedia and in severalothersources. Surely, the motivation for such terminology is clear. I would nevertheless like to know who decided to adpot these (rather amusing) notations, so here is a question:

What was the first paper / textbook that made use of the notations $\flat$ and $\sharp$?

and a possible follow-up question:

If such notations were not adopted widely after the first appearance, who popularized them?

2 Answers
2

Marcel Berger, on p. 696 of his Springer-book: A panoramic view of Riemannian Geometry, writes "Next we define the canonical musical duality", referring to a footnote: "These dualities are called musical because they are often written with symbols like ♭: V $\rightarrow$ V* and ♯: V* $\rightarrow$ V."

Even if this is not the first metioning of "musical", we can assume that Berger invented that expression, because in a panoramic viev of over 800 pages, a reference to another originator could have been expected.

Further Rob Kusner's hint here # or "sharp" is what Marcel Berger calls a "musical isomorphism" and Olivier Bégassat's comment here seem to provide sufficient evidence to determine the inventor.

To answer this question completely, one would have to read through Berger's complete works. But if we wait for a while, this task will probably be reduced to touching a button.

However, I am not convinced that this is the first place where they appear, or that Berger was the inventor. One would have to trace German textbooks in Riemannian geometry from the 1960s or perhaps earlier. Interesting question!